JP2004508723A - Pump source for laser - Google Patents

Abstract

The present invention relates to the field of lasers, and in particular, to pump sources for use in lasers. Many existing lasers use a linear flashlamp to create a population inversion in the gain medium of the laser. Such pump sources have some disadvantages, especially when pumping dye lasers, including explosive damage, long light pulse lengths and inappropriate spectral emission. The present invention provides a pump source for a laser using a surface discharge phenomenon. The dielectric material 1 in contact with the gas 9 has electrical energy discharged on the surface to provide the electromagnetic emission used to pump the gain medium. By varying the dielectric material (among other variables) or the cover gas used, surface emission can be used to pump the laser gain medium.

Description

[0001]
The present invention relates to the field of lasers, and in particular, to pump sources for use in lasers.
[0002]
Lasers operate by creating a so-called population inversion in a material (conventionally called a "gain medium"), which is then stimulated to produce coherent emission of photons. The pump source is the component of the laser that pumps the gain material to the population inversion state, and various lasers utilize various processes to create the population inversion. For example, a typical He-Ne laser uses a high-voltage electric discharge to excite the He-Ne mixture in the discharge tube to a population inversion.
[0003]
A typical flashlamp pumped dye laser comprises several linear flashlamps symmetrically arranged around a central cell containing the dye (gain medium). A reflector surrounds both the dye cell and the flashlamp and directs light into the dye cell (see LG Nair, 'Dye Lasers', Prog. Quant. Electr. For a discussion of the structure of flashlamp pumped dye lasers. , Vol.7, pp153-268, 1982). The most common dye material used is rhodamine 6G dissolved in a methanol solvent. To obtain efficient system operation from a dye laser, a powerful incoherent pump source is generally used. The most common example is to pump the dye laser with a conventional xenon-filled flash lamp. Such devices can operate with efficiencies greater than 1%, but levels of 0.5-1% are more typical.
[0004]
There are several disadvantages to using flash lamps of the type described above as a pump source. First, there is a risk that the flash lamp will explode. When a discharge occurs in the flash lamp, the plasma filament is electrically heated and expands rapidly. This can cause a shock wave to rupture the surrounding silica envelope, which can cause the flashlamp to explode. The risk of this damage means that the level of electrical energy that can be used to drive the flash lamp must be limited. This has a significant effect on the laser efficiency and the output energy of the flashlamp pump laser.
[0005]
Second, the light spectrum from the flashlamp is inherently thermal and is considered a pseudo blackbody with a temperature of about 20,000K. This means that a significant portion of the pump light is in the hard ultra-violet region of the electromagnetic spectrum, which results in significant degradation and heating of the dye solution and poor absorption spectral overlap. (Dye solutions with gain material consist of dye and solvent. Hard ultraviolet radiation from the pump source damages the dye molecules and makes them unable to contribute to laser action. Hard UV photons are also very energetic. If it is absorbed by the dye, it is converted into a large amount of heat, which is dissipated in the dye solution.) Often, in smaller flash lamps, the ablation of the flash lamp itself creates material that contaminates the Xe gas, thereby affecting the emission profile.
[0006]
Third, flash lamps generally have a high initial impedance that limits the energy deposition rate for the discharge. This results in a relatively long electrical drive pulse (of the order of about μs), which can promote the development of significant laser losses, such as large triplet state distribution and / or strong thermal lensing. is there. The triplet state is a significant loss mechanism for dye lasers. When in the triplet state, the dye molecules are not available for laser action and are a source of absorption loss for laser emission. Therefore, when the triplet state distribution is reached, the laser power and energy are reduced. The thermal lensing effect in the dye gain medium causes degradation of the laser beam quality and becomes a mechanism for laser emission loss. The thermal lensing effect occurs when heat is deposited in the dye gain medium and the refractive index changes in the solvent. Thus, the solvent has the properties of a lens.
[0007]
Accordingly, it is an object of the present invention to provide a pump source for a laser that mitigates at least some of the disadvantages described above.
[0008]
According to the present invention, a slab of a dielectric material having at least a part of the surface exposed, and a storage means for keeping a gas in contact with the exposed surface, which can be filled with an appropriate gas. Energy supply means configured to supply energy onto the exposed surface of the substrate during operation of the source, thereby initiating a surface discharge, wherein the impedance of the energy supply A pump source for a laser is provided that substantially matches the impedance of the discharge.
[0009]
Accordingly,
i) keeping the gas in contact with the exposed surface of the slab of dielectric material;
ii) supplying energy on the exposed surface of the dielectric material by an energy supply circuit to initiate a surface discharge;
iii) the impedance of the surface discharge and the impedance of the energy supply circuit are configured to substantially match;
A method is provided for pumping a gain medium for a laser.
[0010]
The present invention utilizes emitted light emission from a surface discharge to provide a laser pump source. Like dye lasers, surface discharges are suitable for use with neodymium glass lasers and photodissociation lasers, which also conventionally use flash lamps as the pumping source. The receiving means provides a cover gas for the configuration and can be filled with a suitable gas. Thus, this allows the user to select his own gas.
[0011]
The device operates by discharging electrical energy onto the surface of a dielectric material (commonly referred to in the art as a "substrate"). The discharge is initiated by starting a number of plasma streams from an energy supply means, conveniently a high voltage electrode. These streams grow from the HV electrode to the ground electrode on the substrate and often merge into the plasma sheet as they develop. As the stream bridges the gap between the electrodes, a high current builds up in the stream, which heats the gas and ablates the substrate, becoming part of the gas. Thus, spectral emissions originate from both the cover gas and the substrate component. The entire process from initial energy supply to radioactive emission is commonly referred to as "surface discharge". Electromagnetic emission from this surface discharge occurs from the infrared wave band to the vacuum ultraviolet wave band. The use of specific substrates and gases can alter the release profile of the present invention. For example, if the emission occurs in the light region, it may be suitable for a laser pumping source for a dye laser.
[0012]
Compared to flash lamps, there is no silica wall, so the device can handle higher input energy. The absence of silica walls that limit surface discharge means that any shock waves emanating from the discharge plasma can dissipate in the gas volume. This is not the case with flashlamps, where silica walls must confine the shockwave. Obviously, the silica envelope cannot contain shock waves, and there is an energy limit at which the silica bursts. This energy limit is called explosive energy. Further, the emitted power has a significant linear structure and is not considered to be thermal.
[0013]
Advantageously, by changing the configuration of the gas, the dielectric material and the energy supply means, the spectrum of the emitted radiation can be changed. This allows the pump source to be adapted to the absorption spectrum of the material used for the gain material.
[0014]
The present invention has lower inductance than linear flashlamp devices due to its flat geometry. Surface discharges typically consist of a number of streams that generally form a planar plasma. The earth return is connected directly to the backside of the substrate. There is no high intensity magnetic field or large circuit loop area associated with this plasma geometry and therefore the inductance is low. Conversely, flashlamps typically contain a single plasma streamer that initially has a small cross-sectional diameter. The return trip is usually a considerable distance from the flashlamp. This creates a strong magnetic field near the streamer and large circuit loop area, which contributes significantly to inductance. In order to ensure efficient energy transfer between the energy supply means and the substrate, it is necessary to ensure that the impedance of the energy supply means substantially matches the impedance of the surface discharge. The impedance of a surface discharge depends on several factors, including the temperature and size of the discharge, and is thus affected by the type of gas used and the area of the substrate where the discharge occurs.
[0015]
If the impedance is low, a short discharge pulse with a high peak power can be generated for an appropriate operating voltage (about 20 kV). This helps to reduce laser losses and the above-mentioned thermal lensing effect. Therefore, it is expected that the surface discharge system will be more efficient because the impedance is lower than for flash lamps.
[0016]
The energy supply means usually comprises a capacitive energy storage device, a transmission line, a switch and a pair of electrodes. A discharge circuit comprising a self-aligned cable pulse generator or a low inductance resonant discharge circuit has been found to be effective. The pulse generator capacitively stores the energy in the high voltage cable that is discharged onto the surface of the substrate during a time determined by the characteristics of the cable. The resonant discharge circuit stores energy in a switchable capacitor on the surface of the substrate.
[0017]
In order to achieve uniform electric breakdown, it is preferable to apply a voltage pulse having a fast rising edge to the HV electrode. This is because a large number of breakdown arcs are thereby formed, and a uniform surface plasma can be formed. Advantageously, a resonant discharge circuit has this property, can discharge energy quickly (about 300 ns), and is more efficient at transferring energy than a cable generator setup. Resonant discharge circuits can drive low impedance loads with high peak power electrical pulses. Thus, the resonant discharge circuit is suitable for driving a low impedance surface discharge load and can produce the high peak power required for a pulsed dye laser pump source.
[0018]
The gas used also has a significant effect on the light emission characteristics of the pump source. The gas type (also gas pressure) affects the uniformity of the discharge. Preferably noble gas (less than 1.5 bar), CO2 ₂ (Less than 0.5 bar) or SF ₆ (Less than 100 mbar) should be used.
[0019]
Small amount of SF ₆ It has been found that the emission activity in the spectral band of 200 to 300 nm is improved when is added to a rare gas (such as Xe or Kr). This effect can be used to increase the pumping efficiency of the dye laser.
[0020]
Conveniently, Xe / SF ₆ Or Kr / SF ₆ It has been found that the use of a PTFE substrate together with produces a particularly strong light emission suitable for pumping dye lasers.
[0021]
Another factor that can affect the structural properties of a surface discharge is the means for making electrical contact between the energy supply means and the substrate. Advantageously, the use of profiled electrodes results in a uniform discharge. A suitably profiled electrode ensures that a uniform electric field is created, and then it is equally likely that a discharge streamer will be formed from any point on the HV electrode edge. Thus, the likelihood of a uniform discharge is increased. For gas discharge lasers, Rogowski or Chang profiled electrodes are commonly used. Chang's electrodes have been tested in our surface discharge system and produce a uniform discharge. However, a simpler configuration that also produces a uniform surface discharge is to use rounded corners for the electrodes. These corners advantageously have a radius of curvature of about 2-5 mm.
[0022]
Surface discharges tend to have low impedance. Therefore, it is important that the inductance of the energy supply circuit is low so that the quality of the driving electric energy pulse does not deteriorate.
[0023]
An energy supply comprising a low inductance discharge circuit having electrodes connected to the dielectric substrate slab by striplines substantially perpendicular to the direction of surface discharge propagation constitutes an intrinsic low inductance energy supply. A ground return in such a configuration can be provided by wrapping the stripline ground conductor around the side of the substrate and returning it to the supply circuit from under the substrate slab.
[0024]
The above configuration, in which the electrical feed is connected at right angles to the discharge propagation, allows the placement of a laser mirror near the edge of the dye slab gain medium, which has the effect of optimizing the energy emission of the laser system. There is a further advantage that there is.
[0025]
Changing the substrate material affects the light emission characteristics of the pump source. Suitable substrate materials for use in the laser pump source are polymers, ceramics and glass. The use of a PTFE substrate has been found to be particularly suitable for pumping rhodamine 6G based dye lasers.
[0026]
The thickness of the substrate is also important. The substrate must be thick enough to withstand the strong shock waves and high electric fields generated during the discharge. However, the substrate must not be too thick, otherwise sufficient capacitive coupling between the discharge and the discharge circuit will not be obtained. Substrates having a thickness in the range of 0.5 to 6 millimeters have been found to promote a uniform surface discharge with sufficient discharge / substrate interaction to modify the spectral emission. For a PTFE substrate, a thickness of about 1 mm has been found to be effective.
[0027]
Spectral emission properties can be further enhanced by including appropriate dopants in the substrate. The doped substrate exhibits strong discrete spectral lines overlying the emission profile of the undoped substrate, and advantageously the dopant material can be selected such that these spectral lines match the absorption band of the gain medium. For example, the use of lead-doped glass as a substrate results in a pumping source that is particularly suited for use with dye-based lasers.
[0028]
Because the pump source of the present invention is planar in geometry, it can be mounted close to the gain medium to optimize direct pumping of the laser. In a further aspect of the invention, there is provided a laser head incorporating a dye gain medium operably coupled to a pump source as described above. Laser heads can have several different configurations. Conveniently, the laser head is a Bethune cell configuration (DS Bethune, "Dye cell design for high-power low-divergence excimer-pumped dye lasers", see Appl. Op. Lateral flow configuration (C Tallman and R Tennant, "Large-scale, excimer-laser-pumped dye lasers" in "High power dye lasers" FJ Duart Ed., 91-Spring, Vol. 91, Springer-91). Can be provided. Preferably, the laser head has a Bethune cell configuration. This is because a more efficient laser can be obtained.
[0029]
The above laser configuration is particularly suitable for liquid dye lasers. However, solid state dye lasers are considered an attractive alternative to liquid dye lasers, and it is therefore a further object of the present invention to provide a pump source configuration suitable for use in solid state dye lasers.
[0030]
Accordingly, the present invention also provides a pump source for a solid state dye laser comprising a pump source wherein the dielectric material can also act as a laser gain material, as claimed above.
[0031]
Such a pump source configuration is useful because there is no need to place a separate liquid dye gain medium nearby. Alternatively, laser action can be achieved from the substrate itself by initiating a surface discharge on the substrate.
[0032]
Suitable substrates include a host substrate doped with a laser dye.
[0033]
Examples of host substrates include solgel, organically modified silica (ormosil), polymers and polymer-filled glass. Examples of suitable dye dopants include organic laser dyes such as pyromethene. In particular, Pyr597 can be used as a dye dopant.
[0034]
The cover gas, substrate thickness, etc. can be changed in the same manner as described above.
[0035]
Next, embodiments of the present invention will be described by way of example with reference to the accompanying drawings.
[0036]
FIG. 1a shows a surface discharge assembly suitable for use as a pump source for a laser. The planar dielectric substrate 1 is connected via electrodes 3, 5 to an energy discharge source (not shown). Electrode 5 is connected to ground plane 7. One surface of the substrate is in contact with the gas 9.
[0037]
When a high voltage pulse is initiated from the electrode 3, an electrical breakdown of the gas 9 occurs in the region between the electrodes 3, 5, as shown in FIG. 1b. Then a surface plasma is formed.
[0038]
FIG. 2 shows a pump source with a substrate / cover gas configuration and a resonant circuit discharge. The figure shows an ALE 402L + 50 kV power supply 21 connected to a 100 nF energy storage capacitor 23. Power supply 21 and capacitor 23 are located in the high inductance (about 2.0 μH) section of the circuit. The inductance is generated by the loop of the wire 25 and the switch 27. This switch 27 comprises a gas-filled in-line triggertron spark gap that separates the capacitor 23 from the rest of the circuit and can block 30 kV when filled with nitrogen at a pressure of one atmosphere.
[0039]
The circuit also comprises a 100 nF low inductance (approximately 4 nH) capacitor 29 made from 0.5 mm thick Mylar® sheet having dimensions of 3 m × 0.45 m and covered with 76 μm copper foil. This sheet capacitor is connected in series with a surface discharge load via a high aspect ratio, low inductance (about 5 nH) stripline (also of Mylar and copper structure). (Note: a stripline comprises two planar electrical conductors with a dielectric insulator between them. The aspect ratio refers to the width of the conductor compared to the thickness of the insulator, ie, a large aspect ratio means a large conductor width and a small insulation Body thickness).
[0040]
The surface discharge is provided by a dielectric substrate made from 1 mm thick PTFE 31. A 20 mm wide copper ground plane (indicated by the dotted line) is connected to the back of the PEFE substrate 31 to minimize surface discharge inductance. The ground plane also forms part of the current return and stripline for the surface discharge.
[0041]
Two copper electrodes 33 having a width of 10 mm, a thickness of 0.45 mm, and a corner radius of curvature of about 2 mm (see FIG. 1 b where the length of the arrow at the upper corner of the electrode 3 represents the radius of curvature of the electrode corner) are formed on the substrate 31 Located above the ground plane. Copper electrodes 33 are 45 mm apart.
[0042]
The substrate 31 has a small amount of SF ₆ (A partial pressure of 800 mbar for Xe / Kr, SF ₆ Is 150 mbar).
[0043]
The discharge circuit was charged to 22 kV. Upon switching the spark gap, about 75% of this energy was transferred to Mylar capacitor 29 and was therefore available for surface discharge.
[0044]
2 was connected to a laser head, and laser energy was measured. FIG. 3 shows that the cover gas is Xe / SF ₆ Or Kr / SF ₆ 4 shows the light pulse shapes for the discharge and laser emission when. The plot in FIG. 3 has been normalized to show pulse length and shape. Plot lines 41 and 42 are respectively Kr / SF ₆ And Xe / SF ₆ 2 shows laser emission using Plot lines 43 and 44 are respectively Kr / SF ₆ And Xe / SF ₆ 2 shows the discharge emission from. FIG. 3 shows how the laser emission responded to the photoexcitation pulse and shows that the laser pulse length was about 400 ns.
[0045]
FIG. 4 shows Kr / SF ₆ (Plot 45) and Xe / SF ₆ Figure 5 shows the spectral energy emission of a surface discharge on a PTFE substrate for (Plot 46). As can be seen, there is significant emission in the light region of the spectrum, thus making this surface discharge configuration suitable as a source for photodye lasers.
[0046]
5a and 5b show two laser head configurations suitable for use with a surface discharge pump source. FIG. 5a shows a Bethune cell configuration. Here, the pump source (substrate 51 and electrodes 53, 55) is located below one surface of a Herasil® silica dye cell 57 having a triangular cross section. Most of the light emitted by the surface discharge is reflected into the silica dye-filled channel 59 by the cell walls. The laser output is indicated by arrow 61.
[0047]
FIG. 5b shows another laser head configuration, a lateral flow dye laser. In this configuration, the dye flows over the surface of the substrate (as indicated by arrow 63). The laser output is in a direction perpendicular to the dye flow. See arrow 65 (dye will flow in or out of the page in the vertical view of FIG. 5b). In FIG. 5b, the same numbers are used to identify elements of the device that are equivalent to those shown in FIG. 5a.
[0048]
5a and 5b, the dye solution consists of 86 μM rhodamine 6G in a methanol solution. The Bethune cell was observed to operate at a higher level of efficiency than the lateral flow cell.
[0049]
FIG. 6 shows three views of the solid state dye laser head configuration. A Pyr597-doped planar dielectric polymer slab 70 (modified poly (methyl methacrylate) polymer-MPMMA) is connected to the discharge circuit 76 via electrodes (72, 74). The load is separated from the discharge circuit by a nitrogen-filled triggertron spark gap 78.
[0050]
The substrate is covered by gas 80 (0.5 bar xenon). The strip lines and electrodes have a low impedance configuration. That is, the return path from the ground electrode 74 to the discharge circuit is on the back surface of the substrate (ground return path 84). At one end of the slab is a 100% reflective mirror 86, and at the other end is an output coupler 88. The distance between the 100% mirror and the output coupler is minimized by configuring the stripline perpendicular to the surface discharge propagation and laser emission direction 82.
[0051]
PTFE reflector 90 and PTFE sheet 92 are located above and below the substrate, respectively, to maximize the coupling of the discharge light into the gain medium. Unmodified PMMA is used on both sides of the dye slab 94 as an insulating sheet.
[0052]
The substrate 70 shown in the figure had a length of 14 cm, a width of 5 cm and a thickness of 4 mm. Laser dye Pyromethene 597 (Pyr597) has a concentration of 10 ^-4 M is mixed in the substrate. (Note: For efficient pumping of the laser dye, the surface discharge must have a strong emission in the laser dye absorption band. Since the dye is doped into MPMMA transmitting only above about 290 nm, Only the visible absorption band of Pyr597 is pumped (470-554 nm)).
[0053]
The electrodes are 11 cm apart and have a corner radius of about 2 mm and a width of 2.5 cm. Electrode separation limits the discharge length to 11 cm. The discharge circuit is designed to supply 100 J of electrical energy to the surface discharge load in about 80 MW pulses. A surface discharge with this energy load, power load and discharge length results in a discharge impedance of about 0.5Ω, so the discharge circuit is designed as a matched 0.5Ω lumped element pulse forming network (pfn). The circuit consists of five parallel LC elements with a total storage capacitance of 1.14 μF. When using a 0.5Ω load, the pulse length is designed to be 1.25 μs.
[0054]
The circuit was charged to 13.3 KV, corresponding to a peak power of 80 MW for a 100 J capacitive stored energy, thus 0.5 Ω load. To minimize stray circuit inductance that distorts the electrical pulse shape, circuit components are connected 0.5 mm apart with Mylar dielectric layers using a low inductance copper stripline 10 cm wide.
[0055]
A ground stripline located below the dye slab (84) provides capacitive coupling to the discharge through the substrate.
[0056]
FIG. 7 compares the light pulse shapes of the laser output 98 and the surface discharge 96. The light pulse length was 1.9 μs with a 6.5 μs tail. The laser pulse was 950 ns shorter than the expected surface discharge emission pulse.
[0057]
The maximum energy value of the laser was 69 mJ obtained for an output coupler reflectivity of 50% (see FIG. 8). For the laser pulse shown in FIG. 4, this gave a power output of 73 kW.
[0058]
One skilled in the art will readily appreciate that other configurations of the pump source are considered to be within the scope of the present invention. For example, for the configuration shown in FIG. 6, a dye-doped polymer with lower absorption at the lasing wavelength could be used. The surface discharge can also be modified to ensure that the laser dye emits more in the visible absorption band.
[Brief description of the drawings]
FIG. 1a
FIG. 4 illustrates a representative surface discharge assembly.
FIG. 1b
FIG. 4 illustrates a representative surface discharge assembly.
FIG. 2
FIG. 4 is a diagram showing a layout including a resonance discharge circuit and a surface discharge platform.
FIG. 3
FIG. 3 shows a normalized light emission intensity profile of the layout shown in FIG. 2 comparing laser emission with discharge emission with different gases.
FIG. 4
FIG. 3 shows normalized emission spectra of gas types in the different types of devices shown in FIG.
FIG. 5a
FIG. 2 schematically shows different laser dye heads (the figure shows two laser heads in transverse and longitudinal section, in each case the transverse section is along the line 2-2 shown in longitudinal view); .
FIG. 5b
FIG. 2 schematically shows different laser dye heads (the figure shows two laser heads in transverse and longitudinal section, in each case the transverse section is along the line 2-2 shown in longitudinal view); .
FIG. 6
FIG. 3 is various views illustrating a surface discharge pump dye laser head and discharge circuit according to a solid state dye laser pump source embodiment.
FIG. 7
FIG. 7 shows the light pulse shapes of (96) laser emission (98) and surface discharge light emission resulting from the setup of FIG.
FIG. 8
FIG. 7 illustrates the laser energy resulting from the setup of FIG.

Claims (23)

A slab of dielectric material having at least a portion of the surface exposed, and a containment means capable of being filled with a suitable gas for keeping said gas in contact with said exposed surface; during operation of the source Energy supply means configured to supply energy onto the exposed surface of the substrate and thereby initiate a surface discharge, the pump source for a laser comprising: A pump source for a laser whose impedance substantially matches the impedance of said surface discharge. The pump source according to claim 1, wherein the energy supply means comprises a self-aligned cable pulse generator. The pump source according to claim 1, wherein said energy supply means comprises a low inductance resonant discharge circuit. 4. The pump source according to claim 3, wherein the discharge circuit is capable of discharging energy within a few hundred nanoseconds. 5. The pump source according to claim 1, wherein the storage means is filled with a rare gas having a pressure of less than 1.5 bar. Pump source according to claim 5 containing SF ₆ of the gas further small percentage. The pump source according to any one of claims 1 to 6, wherein the cover gas includes carbon dioxide. A pump source according to any of the preceding claims, wherein the energy supply means is operatively connected to the dielectric material by a profiled electrode. 9. A pump source according to any of the preceding claims, wherein the energy supply means comprises a stripline electrode connected to the substrate so as to be substantially perpendicular to the direction of surface discharge propagation during use. The pump source according to any one of claims 1 to 9, wherein the dielectric material is a polymer, ceramic or glass material. The pump source according to claim 10, wherein the dielectric material is PTFE. A pump source according to any preceding claim, wherein the thickness of the dielectric material is in the range of 0.5 to 6 millimeters. A pump source according to any of the preceding claims, wherein the dielectric material is doped. A pump source for a solid-state dye laser comprising a pump source according to any of the preceding claims, wherein the slab of the dielectric material is capable of acting as a laser gain material. The pump source according to claim 14, wherein the dielectric substrate material is solgel, an organically modified silica (ormosil) polymer, or a polymer-filled glass. The pump source according to claim 14, wherein the dielectric material is doped with a laser dye. 17. The pump source according to claim 16, wherein the laser dye is pyromethene. The pump source according to claim 17, wherein the laser dye is Pyr597. A pump source according to any of claims 14 to 18, wherein the thickness of the dielectric material is in the range of 0.5 to 6 millimeters. 14. A dye laser comprising: the pump source according to claim 1; and a laser head including a dye gain medium, wherein the laser head has a Bethune cell configuration. A dye laser comprising a pump source according to any one of claims 1 to 13 and a laser head comprising a dye gain medium, wherein the laser head has a lateral flow configuration. 20. A dye laser comprising a pump source according to any of claims 14 to 19, and a laser head comprising a PTFE reflector disposed above the substrate and a PTFE sheet under the substrate. i) keeping the gas in contact with the exposed surface of the slab of dielectric material;
ii) discharging energy onto the exposed surface of the dielectric material by an energy supply circuit to initiate a surface discharge;
iii) the impedance of the surface discharge and the impedance of the energy supply circuit are configured to substantially match;
A method of pumping a gain medium for a laser.

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